652 THE SENSE OF SIGHT. 



to be formed at a point anterior to the retina : the defect 

 is remedied by the use of concave glasses. Presbyopia, or 

 long-sightedness, is the result of conditions the reverse of 

 the above, and is remedied by the use of convex glasses, 

 which diminish the focal distance of an image formed in 

 the eye.* 



3. The direction given to the rays by their refraction is 

 regulated by that of the central ray, or axis of the cone, 

 towards which the rays are bent. The image of any 

 point of an object is, therefore, as a rule (the exceptions to 

 which need not here be stated), always formed in a line 

 identical with the axis of the cone of light, as in the line 

 of B a, or A 1}, fig. 185 : so that the spot where the image 

 of any point will be formed upon the retina may be deter- 

 mined by prolonging the central ray of the cone of light, 

 or that ray which traverses the centre of the pupil. Thus 

 A b is the axis or central ray of the cone of light issuing 

 from A ; B a, the central ray of the cone of light issuing from 

 B ; the image of A is formed at b, the image of B at a, in 

 the inverted position ; therefore what in the object was 

 above is in the image below, and. vice versa, the right-hand 

 part of the object is in the image to the left, the left- 

 hand to the right. If an opening be made in an eye at 

 its superior surface, so that the retina can be seen through 

 the vitreous humour, this reversed image of any bright 

 object, such as the windows of the room, maybe perceived 

 at the bottom of the eye. Or still better, if the eye of any 

 albino animal, such as a white rabbit, in which the coats, 

 from the absence of pigment, are transparent, is dissected 

 clean, and held with the cornea towards a window, a very 

 distinct image of the window completely inverted is seen 

 depicted on the posterior translucent wall of the eye. 

 Volkmann has also shown that a similar experiment may 



* For details on this subject, consult the various treatises on the 

 Physiology and Defects of Vision. 



